Numerous technologies require the uniform deposition of a variety of metals from the vapor phase in controlled amounts. One such technology is the preparation of light-emitting diodes and other photonic devices by the doping of Group 13 nitrides by magnesium. Magnesium doping of GaN devices is theoretically capable of providing stable, high-luminosity blue and green photonic devices. Magnesium is particularly useful as a dopant due to its low diffusion constant in semiconductor matrices, its desirable acceptor energy level, and the relatively low toxicity of its compounds.
Thus far, the magnesium sources used in such techniques as metal organic chemical vapor deposition (MOCVD), chemical beam epitaxy (CBE), and related techniques have employed bis(cyclopentadienyl)magnesium (Cp.sub.2 Mg) and substituted derivatives. However, Cp.sub.2 Mg is a solid of high melting point and low vapor pressure. Moreover, the cyclopentadienyl ligands of Cp.sub.2 Mg are known to bond strongly to metals, their cleavage from which are relatively high energy processes. The low vapor pressure and low reactivity of Cp.sub.2 Mg and like compounds cause a so-called "memory effect" during growth of doped films, where Cp.sub.2 Mg adheres to the walls of the reactor, only to slowly desorb following cessation of dopant precursor flow. This process temporarily lowers dopant concentration initially, and prolongs dopant availability following the desired end point. Thus, a broad, rather than an abrupt doping profile is created. The memory effect places a severe impediment on the preparation of devices with well defined doping profiles.
Despite the known inadequacies of Cp.sub.2 Mg as a dopant source, only very few attempts have been reported to alleviate such problems. Hatano et al., APPL. PHYS. LETT., 1991, 58, 1488 have employed Mg(Al(CH.sub.3).sub.4).sub.2 to prepare doped Ga.sub.x Al.sub.1-x As films at high doping levels and with flat doping profiles, thus exhibiting virtually no memory effect. However, the presence of Al and the facile reversion of this compound to the constituent metal alkyls prevents its use for magnesium doping.
CVD precursors for depositing Group IIa metal are disclosed in related U.S. Pat. Nos. 5,280,012; 5,225,561; and 5,453,494. Calcium, barium, and strontium precursors of the bis(cyclopentadienyl) or acetylacetonate-type complexed with a mono- or multidentate ligand are disclosed for use in preparing doped copper oxide superconductors. However, no magnesium precursors are exemplified, and the Group IIa complexes prepared exhibited relatively high sublimation temperatures, higher than Cp.sub.2 Mg.
It would be desirable to provide Group IIa metal-containing, and particularly magnesium-containing organic source compounds having high volatility which can be used as MOCVD precursors. It would further be desirable to provide magnesium MOCVD precursors which exhibit little or no memory effect.